Display screen or portion thereof with a graphical user interface



The FIGURE is a front view of a display screen or portion thereof with a graphical user interface showing an image of my new design.

The outermost dot-dash broken lines in the FIGURE show a display screen or portion thereof and form no part of the claimed design. The other broken lines in the FIGURE show portions of the graphical user interface that form no part of the claimed design. The dotted lines within the instrument tray section of the claim are ornamental and form no part of the claim. 

CLAIM The ornamental design for a display screen or portion thereof with a graphical user interface, as shown and described. 